h may be straight chained or branched provided that only one of R1 to R6 can be an alkyl group, with the proviso that when p, q and r have a value of 0 and Y is oxygen, m+n is at least 11, and with the further proviso that when X is an imino group, q is equal to 1, Y is oxygen, and p and r mare O, then m+n is at least 11, will enhance the rate of the passage of the drug across body membranes.